1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming system, and a density unevenness correction method.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet, and then heat and pressure are applied to the sheet at a fixing nip to form an image on the sheet.
In such an image forming apparatus, the image quality of an output image (image formed on a sheet) may be degraded due to degradation over time of a photoconductor drum, a developer, or the like, the environment around the apparatus (changes in temperature and humidity), or the like. Specifically, an output image may not be faithfully reproduced based on the color of an input image, or tints may differ between images in some situations. As such, in conventional image forming apparatuses, image stabilization control is performed in order to ensure color reproducibility and color stability.
Further, in an image forming apparatus, density unevenness in a circumferential direction (sub-scanning direction) may be caused in a toner image formed on a photoconductor drum due to distance variation between the photoconductor drum and the developing roller caused by rotational runout of a developing roller. In that case, density unevenness is caused also in an image formed on a sheet in synchronization with the rotational cycle of the developing roller. In the image stabilization control for preventing such cyclical density unevenness, density of a patch image (toner pattern) formed on a photoconductor drum is detected by an optical sensor, and density correction of an image is performed by performing image processing on an input image data based on the detection result, or changing image formation conditions such as charging potential, developing potential, and a light exposure amount. In general, image stabilization control is performed regularly using a non-image formation region when image formation is performed continuously on a plurality of sheets.
Japanese Patent Application Laid-Open No. 2013-88717 discloses a technique of performing more preferable correction by predicting the amplitude of banding (horizontal streaks caused by density difference) at the time of printing, and performing banding correction processing based on the predicted amplitude. The image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2013-88717 includes a developing roller configured to perform periodic movement for formation of an image, a table holding unit configured to hold a table for correcting density variation caused by electrical resistance of the developing roller, which is created in a reference state of the developing roller, a prediction unit configured to predict amplitude of variation in a state different from the reference state, and an adjustment unit configured to adjust the table based on the amplitude predicted by the prediction unit.
However, in a state immediately after turning on and activation of the image forming apparatus after the image forming apparatus has been stopped for a long time (hereinafter referred to as “post-stop state”), the amount (hereinafter referred to as “developer conveyance amount”) of the developer conveyed on the developing roller is large and unstable due to decrease in the charge amount of the developer. The developer conveyance amount is decreased as the time passes from the post-stop state, but is stabilized at a certain level in a steady state after continuous printing. Consequently, in the post-stop state where the developer conveyance amount is large, the density unevenness in the sub-scanning direction and accordingly the density correction amount required for the density unevenness are increased in comparison with those in the steady state. FIG. 1 illustrates density of a patch image detected at corresponding rotational positions of the developing roller in the post-stop state and in the steady state. As shown in FIG. 1, an amplitude B of a waveform (dotted line in the figure) representing a density change of the patch image in the post-stop state is larger than an amplitude A of a waveform (solid line in the figure) showing a density change of the patch image in the steady state. Therefore, the density correction amount required for correcting the density of the toner image to be a target density in the post-stop state is larger than the density correction amount required for correcting the density of the toner image to be the target density in the steady state. As such, the correction amount may disadvantageously become excessive and density unevenness in the sub-scanning direction may not be corrected when the density correction is continued using the density correction amount calculated in the post-stop state as it is.
It should be noted that the technique described in Japanese Patent Application Laid-Open No. 2013-88717 is not a technique intended to control a transfer voltage which should be applied to a transfer member in accordance with changes in the image formation environment during the image formation processing regardless of the image formation conditions, and as such the technique does not include a configuration for that purpose.